Kerbodyne SSTO Division: Omnibus Thread

[Wanderfound]

Just FYI, but the slideshow doesn't show Wanderfound's text explanations when in slideshow mode. To see them, turn off the slideshow and go through the pictures manually.

Thanks, hopefully fixed now.

[Wanderfound]

BTW: if anyone's curious to see the final version of the ship from the tutorial, here it is:

https://www.dropbox.com/s/rp0m3grjm45yqa1/Kerbodyne%20Simplicio.craft?dl=0

Just a few changes after the test flight...

Added the forgotten solar to the satellite, and removed the non-stock life support parts. Added some more batteries as well.

Extra landing gear. As well as extra toughness and stability, this gives us extra brakes; needed. The RCS was doubled, which should provide better translation balance and more landing retro-thrust. The rearmost normal gear were also shifted slightly rearwards, in order to steepen the pitch angle between the rear gear and the tailstrike guards. This should hopefully ease the takeoff difficulty slightly.

As should this. Instead of having all of the airbrakes linked to the landing brakes, they are now split in half, controlled by action groups 9 and 0. This allows the lower half of the airbrakes to be deployed independently as flaps.

However, the #1 way to ease takeoff also applies: more speed, more thrust. Just run the LV-N during takeoff and it's fine.

See? Easy.

Edited November 22, 2014 by Wanderfound

[Wanderfound]

Like your VTOLs, but don't want to be limited to oxygenated worlds? Sick of waiting around for nuclear rockets to finish their burns?

Come fly the new Kerbodyne Cacafuego. There's nowhere it can't go.

(note: Kerbodyne accepts no liability for damage done by VTOL exhaust to runways, helipads, forests, housing or biospheres. Look before you land.)

Easy and stable VTOL performance. Instant-response rocket thrust provides a safety margin unavailable to jet-lifted craft.

Smoothly converts to horizontal flight.

Climbs well in either mode.

Simple to orbit.

Use VTOL Aerospikes for vacuum propulsion in order to enhance fuel efficiency.

Interplanetary range provided through the use of drop tanks. These tanks are narrow enough to be economically and easily lifted to orbit by spaceplane, and the linear and trailing placement of the tanks removes any issues with docking port misalignment or flexibility. As the tank attachment point lies directly upon CoM, the train of tanks can be extended indefinitely without upsetting the vehicle balance.

Dedicated science explorer microsat. Sufficient RCS on board the mothership to refuel the microsat several times over.

Not a lot of fuel, but useful delta-V and impressive TWR (take a look at the G meter); you don't need a lot of thrust on a vehicle this lightweight. Drop off the probe on your way into a system and recollect it on the way out, or use it as an advance coalmine canary when planning an aerobrake.

Very stable in the hover.

Keep the Vernors on at all times while hovering; they're your primary stability aid.

Weight balance is vital in any VTOL, but the lack of Aerospike gimballing ability makes it doubly so in this case. Keep your fuel load balanced, and tune the VTOL thrust by slightly adjusting the position of the forward thruster if you change anything in the SPH.

Craft file at https://www.dropbox.com/s/1ovzen4998vylg2/Kerbodyne%20Cacafeugo.craft?dl=0

Edited November 23, 2014 by Wanderfound

[eddiew]

Okay folks; I've put together a little slideshow tutorial. It goes all the way through the construction of a spaceplane, to taking it to orbit, bringing it through reentry and landing back at KSC. There are just under 200 slides, but they're all JPEGs so it shouldn't be too bandwidth hungry.

This was brilliant, btw, really useful in sorting out my design process. An evening of thoughtful planning, and I got a small SSTO from ground to orbit in two tries (first time I wasn't aggressive enough on the ascent and wasted fuel faffing around at 15k for ages)

Although doing a rescue contract, then getting back down with 8m/s left in the tank and only just stopping at the very end of the runway was a bit... close. Never expected to be using the emergency landing chute after landing Maybe next time I'll swap the docking port for a small fuel tank - didn't really need to dock with anything anyway so it was just acting as forward weight. Worst case, a KAS winch in the cargo bay would allow a fuel transfer.

Maybe tonight I'll try to get a satellite up the proper way

[Wanderfound]

Did somebody tell you that Mk2 cargo bays are too small to hold anything useful?

Stop listening to that person. They don't know what they're talking about.

Instead, come fly the new Kerbodyne Investigator. Long range, ample performance and two integrated exploration vehicles: probe and rover. Deployable, recoverable, highly capable. Explore the solar system for nothing more than the cost of the fuel.

Simple lift off.

And easy climb performance.

Stable under time acceleration.

Cruise to orbit on the nuke.

5,000m/s of ÃŽâ€V still in the tanks.

Time for the first trick.

Release the drone.

A full science payload, 10G thrust and a decent chunk of ÃŽâ€V. Sufficient RCS reserve on board to refill the probe several times over.

Use of thrust limiters recommended for fine manoeuvring; even at minimal throttle, this thing has a lot of kick. Action group 9 will disable the payload RCS during flight, while action group 8 toggles the probe's monoprop propulsion.

Nuclear propulsion makes Munar or interplanetary travel easy. RAPIERs remain in reserve in case of the need for emergency thrust during landing.

Vernors to soften the touchdown; simple to land in air or vacuum.

And now for the second trick; open the payload doors...

...and drop the cargo.

Rovey goodness.

Able to self-right with RCS if flipped. Onboard probe core permits unKerballed operation.

To recover: first park your rover near the rear of the bay.

Raise the rover on its landing struts.

Before lowering the plane's landing gear.

Precise alignment not required.

Use the rover's RCS to push up the last few inches. The rover has sufficient thrust to fly even in full Kerbin gravity. Retract the struts after docking, and redeploy the mothership's landing gear.

Not bad aesthetically, either.

Craft file at https://www.dropbox.com/s/akf8r1akrqoq1wy/Kerbodyne%20Investigator.craft?dl=0

[goldenpsp]

For vids have your given open broadcaster a try? It is free and while designed for streaming it can also record.

[Somtaaw]

BTW, what you've got looks okay, although we'd need to see CoL to judge. In stock, you want CoL overlapping CoM but not in front of it.

The pure Aerospike propulsion would make it difficult to reach orbit before running short of fuel. RAPIERs would make it a lot easier. I'd recommend adding a docking port for orbital refuelling (unless that piece behind the passenger cabin is one?).

The piece directly behind the crew cabin is a docking port, angled downwards. I was aiming somewhat for a spaceplane similar to the 'Armageddon' Bruce Willis movie, the X-71, which had side mounted docking rings.

I went ahead and grabbed FAR, and the only gripe I have about it, is a few of my key re-entry rockets come in hard and fast, and alot of my internally mounted batteries, RNGs, MechJeb control panel, etc get torn off by sheer stresses, and since some of them rely entirely upon MJ tweaking the re-entry to land exactly at KSC thats a mild problem, compared to getting my Spaceplanes to handle properly.

And I had indeed gone for RAPIERs, although I had kept the Aerospikes, originally it looked like they had good performance, but the more I browse spaceplane threads, the less I like them. I had a nice Mk2 jet, but then I accidentally deleted it so I'm re-assembling it, and bringing in some of the changes I liked from Ashlain's Aquila enterprise aircraft.

I'll upload a picture and craft file, for your expert FAR lookover, right after I finish a reboot. Installed Dropbox on my desktop and laptop to better transfer my KSP between them.

[Wanderfound]

To protect stuff from the wind in FAR, you need to do it properly: fairings and cargo bays. Just part clipping it under the surface won't work; FAR will still treat it as externally mounted. If you right click on parts, you should see a message saying "shielded=false" or "shielded=true". If it ain't shielded, then it's subject to aerodynamic failure.

It's possible to build a rocket that can cope with hitting the lower atmosphere at Mach 6, but it's usually not worth the bother. If you're not in a spaceplane, you generally want to be reentering in a bare capsule. Especially if DRE is in play.

[Somtaaw]

My shuttle, meets extended range probe launcher, meets KSC lander mark 3. I still have to add in all the rcs thrusters, and possibly a tail assembly, but also uploading the .craft file so its easier for you to manipulate.

https://www.dropbox.com/s/jolxda8n0ggqbxp/OOPS%20Mk_3.craft?dl=0 - the shuttle itself

https://www.dropbox.com/s/4c262ao41fmtqux/Probe%201.5m.craft?dl=0 <- a probe I've been trying to get to attack to the cargo bay's clamps.

A note, that the cargo bay's clamps (both jr and medium) are both air attached to the cargobay itself, which might be the reason I'm unable to actually get the 2 to connect.

[Wanderfound]

My shuttle, meets extended range probe launcher, meets KSC lander mark 3. I still have to add in all the rcs thrusters, and possibly a tail assembly, but also uploading the .craft file so its easier for you to manipulate.

http://i.imgur.com/p15cBtp.png

https://www.dropbox.com/s/jolxda8n0ggqbxp/OOPS%20Mk_3.craft?dl=0 - the shuttle itself

https://www.dropbox.com/s/4c262ao41fmtqux/Probe%201.5m.craft?dl=0 <- a probe I've been trying to get to attack to the cargo bay's clamps.

A note, that the cargo bay's clamps (both jr and medium) are both air attached to the cargobay itself, which might be the reason I'm unable to actually get the 2 to connect.

Looking okay so far. The cargo bay docking ports should work where they are, although turning off radial attachment while mounting the payload would make it easier.

The first tweak I'd do is to shift the lateral tanks forwards until CoM and dCoM coincide. It also needs some sort of vertical stabiliser.

Edited November 25, 2014 by Wanderfound

[blowfish]

Ideally the CoM, dCoM and payload CoM should all line up so that you don't get weight shifts in flight. Of course getting this to work can be tricky sometimes...

[Wanderfound]

My shuttle, meets extended range probe launcher, meets KSC lander mark 3. I still have to add in all the rcs thrusters, and possibly a tail assembly, but also uploading the .craft file so its easier for you to manipulate.

http://i.imgur.com/p15cBtp.png

https://www.dropbox.com/s/jolxda8n0ggqbxp/OOPS%20Mk_3.craft?dl=0 - the shuttle itself

https://www.dropbox.com/s/4c262ao41fmtqux/Probe%201.5m.craft?dl=0 <- a probe I've been trying to get to attack to the cargo bay's clamps.

A note, that the cargo bay's clamps (both jr and medium) are both air attached to the cargobay itself, which might be the reason I'm unable to actually get the 2 to connect.

Okay: see https://www.dropbox.com/s/1betmydkrsun5ed/OOPS%20Mk_3K.craft?dl=0 for a Kerbotuned version. I tried to keep the aesthetics and basic design concept as close to the original as possible while still getting the aerodynamics sorted. If you hate the canards, you could lose them, but you'd need to shift the wings forwards in order to compensate.

The main trick to attaching payloads is just to disable surface attachment (hold down the Alt key) while you're doing it. I was unable to load your probe because it has a part on it from a mod I don't use. Instead, I put a few of my own satellites and probes into the bay as demonstrators.

I replaced the rear lateral tanks with nacelles for two reasons. Firstly, it was a bit short on intakes (a rarity, normally people err much too far the other way). Secondly, you always want a touch more LF than O on a spaceplane, because you're going to be burning pure LF on the way up. If you have an LFO fuel mix perfectly balanced for 100% rocketry, you're always going to find yourself with a little bit of useless O after the LF runs out.

In order to keep it friendly to less experienced pilots, I kept the wings a bit stronger/heavier than I would normally do, and biased the aero a touch towards stability over agility. I've also added the usual final polish; action groups, electrics, RCS etc. I've locked off the probe core battery and the cockpit RCS as emergency reserves; right click to reactivate if needed.

Good for takeoff; if you look at the "level flight" numbers at top right, you'll see that it needs less than ten degrees of AoA to lift off at Mach 0.35 (which is a speed it reaches about halfway down the runway). The flaps are not needed for takeoff, but I've left them enabled because you might want them to permit slower landings.

All good at the top of the jet range.

And all nice in the bottom of the rocket range, too.

Very stable even at extreme AoAs (the downward-angling Cm line is the key thing here).

Off we go; raise the flaps to minimise drag during the takeoff run.

And up. Very easy liftoff.

Climbs easy, too. I tested it out and it's tough enough to cope with 10G manoeuvring down at treetop height, but it's still a good idea to start with a steep climb. This thing has enough TWR to go supersonic fairly quickly, and that's always a dangerous thing at low altitude.

Level off as soon as the air thins.

And crank it up to speed. Smoothly passes the time acceleration test.

As soon as the engines switch to rocket mode, toggle off the outer pair (action group 2) and flick the inner pair back to jet mode (action group 3).

Climb until the inner pair switch to rocket mode again, then close the intakes (AG5) and reactivate the outboard engines (AG2).

Keep the nose down to raise your periapsis.

A hyper-flat trajectory like this does see you lose a bit of speed to drag after engine shutdown, but it should be more than made up for by the reduced gravity losses.

And the flat trajectory allows for tiny circularisation burns.

A distinctive silhouette; Wanderfound like.

Pop out the Fine Print satellites...

Look at the G-meter; these little things have an insane amount of kick.

So much, in fact, that they can hit escape velocity from LKO with their tanks still half full.

I also included a similar monoprop-powered recoverable science probe.

Not quite as fast as the other one, but still not slow. This is the only screenshot I had time to take while both ships were still in view.

And also able to reach escape velocity with just a tiny expenditure of monoprop.

Edited November 26, 2014 by Wanderfound

[Jovus]

What are the 'Pump Level', 'Pump Options' and so forth in the last picture?

Are you really going to make me pick up another mod?

[Wanderfound]

What are the 'Pump Level', 'Pump Options' and so forth in the last picture?

Are you really going to make me pick up another mod?

Goodspeed. Like TAC-FB, but better.

Goodspeed allows you to set every tank with a "pump level" between one and eight. If they're set to auto-pump, then fuel will automatically flow "downhill" towards the tanks with lower pump levels.

This is brilliant for orbital refuelling; set the refuelling station to five, your fuel delivery tankers to six and your normal craft to four. As soon as you dock, fuel will start to flow from the tankers to the station or the station to the non-tankers. You can set the pump levels of tanks individually, so you can also use this to control fuel flow within a single ship.

It's by no means compulsory, but it's very nice. And it's another of those tiny utility mods that take next to zero RAM and are unlikely to cause crashes. The right-click options are like the FAR control surface tweakables; if you design a craft with the mod, then load it without the mod, all that happens is that the special fuel pumping options no longer appear.

[bakanando]

I have to say thanks to wanderfound for all SSTOs here, even though I don't use FAR/NEAR and have only used stock the spaceplanes here have given so many ideas and the tutorials have made spaceplane construction my first means of exploring the kerbol system. Last week I hit a milestone: my first spaceplane to munar orbit (but not yet back).

Keep up the great work you've been doing.

[Wanderfound]

BTW, just for comparison, the original aero analyses [1] of OOPS Mk3, in high-res so you can read them.

With all of these analysis numbers, the moment they go from green to red is the moment that they pass zero. But, for some things, negative is good, while for other things positive is good. And there is such a thing as too much of a good thing; an overly stable plane is a non-manoeuvrable lawn dart. So, small numbers good, large numbers bad.

So, Mach 5 at 25,000m; about where your jets max out.

Tends to flip itself over aggressively in the roll axis when its sideslipping too much. Increase wing length or add some dihedral.

Should straighten out aggressively when sideslipping hard, but doesn't. Add more vertical stabiliser.

Tends to slip sideways at an increasing rate when you're trying to level the wings. Dihedral and vertical stabiliser.

This one is saying that the faster you straighten out, the more you should sideslip (it's the inverse of the second one). Again, vertical stabiliser (AKA a big tailfin, as far back as possible).

This is saying that there's a tendency to pitch-up as Z-direction (down) velocity increases. Keep in mind that a plane doesn't have to diving in order to have a high Z-velocity (actually, diving reduces your Z-velocity because of the reduced AoA). Because of the AoA required for lift, planes are always flying slightly down relative to the axis of the plane. Either add some more tailplane (horizontal stabilisers, again as far back as possible) or remove some lift from the front of the craft.

[1] Sontaaw, I hope you don't take this the wrong way. It's not intended as a criticism of the original design; I just thought that it made a useful teaching moment. The original was much closer to flyable than most stock-aero based designs are.

Edited November 27, 2014 by Wanderfound

[Wanderfound]

Greenpeace on your case after you crashed that nuclear rocket into the middle of town? Or do you just hate waiting around for 20 minute LV-N burns all the time?

Want to go interplanetary, and want to do it cheap?

Get yourself down to the local Kerbodyne showroom and check out the new Impatienze. Unlimited range, plenty of room for science gear and probes, all combined with 350kN of express delivery thrust.

Just don't tell the Greenpeace guys about the PB-Nuk's stashed in the cargo bay.

Craft file at https://www.dropbox.com/s/a2ugb916a4xgs2f/Kerbodyne%20Impatienze.craft?dl=0

The usual easy Kerbodyne launch.

Power up to orbit as usual. No need for the hyper-shallow LV-N approach, although as always a more jet-focussed flight will be more fuel efficient.

Only 1,300m/s of delta V in the tanks? How could we possibly go interplanetary with that?

Like this:

...and there's another 1,000m/s of delta V.

Because the tanks are connected in a linear fashion, in line with CoM, there's nothing to stop you from making the line of drop tanks arbitrarily long. Launch the ship, leave a tank in orbit, fly back to KSC, rinse and repeat. Each tank should only cost a couple thousand Kerbucks to put into orbit. Once you've got the last one up, fill the cargo bay with as much science gear, probes, landers and rovers as you like, then rendezvous with your drop tanks and head off to whichever planet you choose.

By default, the batteries and fuel on the drop tanks are locked off; they don't have solar panels, so leave the batteries deactivated until it's time to connect them to the mothership. While you're putting the drop tanks into place, there's no need to worry about fuel efficiency; feel free to climb steep and fast, and don't worry if the RAPIERs switch modes while you're still only at Mach 4.

Standard Kerbodyne aero polish.

Edited November 27, 2014 by Wanderfound

[Jovus]

I'll chip in and say that the anteprevious post was a valuable teaching moment for me, at least. It's nice to see a 'and here's what you do about it' rundown for aero problems, so thanks.

[mattinoz]

Curious with the "Impatienze" is there any reason why you wouldn't put the cargo bay to open down?

Was thinking when the secret engine could slide in place without working around the tail.

[Wanderfound]

Curious with the "Impatienze" is there any reason why you wouldn't put the cargo bay to open down?

Was thinking when the secret engine could slide in place without working around the tail.

Down or up wouldn't make much difference; there is no secret engine. The tail of the plane carries a docking port; the drop tanks are just tanks. Their only propulsion is RCS.

You could make the final pre-departure payload into a dockable LV-N instead, but then you'd have to deal with the wobbly docking port issue. With the drop tanks as just an unpowered trailer, the thrust from the RAPIERs pulls them straight instead of pushing them wobbly. In theory, there is no limit to how many tanks you can hang off the back.

A down facing bay would avoid having to go over the tail, but getting the tank from the bay to docked in the rear only takes a few seconds anyway. Decouple, tap K for RCS up, then N for a bit of back, then I and H to bring it in to the docking port. A single drop tank is light enough that docking is very easy; get anywhere close and the magnets will pull it into place.

For a long range multi-tank mission, I'd dock each new tank to the growing collection of docked tanks in orbit, then do the final attachment while controlling the mothership from the rear docking port.

But feel free to flip the bay if you like; it shouldn't affect the balance of the ship.

[Somtaaw]

BTW, just for comparison, the original aero analyses [1] of OOPS Mk3, in high-res so you can read them.

With all of these analysis numbers, the moment they go from green to red is the moment that they pass zero. But, for some things, negative is good, while for other things positive is good. And there is such a thing as too much of a good thing; an overly stable plane is a non-manoeuvrable lawn dart. So, small numbers good, large numbers bad.

So, Mach 5 at 25,000m; about where your jets max out.

Tends to flip itself over aggressively in the roll axis when its sideslipping too much. Increase wing length or add some dihedral.

http://i1378.photobucket.com/albums/ah120/craigmotbey/Kerbal/Tuneups/screenshot114_zps3b1d61e6.png

Should straighten out aggressively when sideslipping hard, but doesn't. Add more vertical stabiliser.

http://i1378.photobucket.com/albums/ah120/craigmotbey/Kerbal/Tuneups/screenshot115_zps2b3c9d42.png

Tends to slip sideways at an increasing rate when you're trying to level the wings. Dihedral and vertical stabiliser.

http://i1378.photobucket.com/albums/ah120/craigmotbey/Kerbal/Tuneups/screenshot116_zps6f335ae9.png

This one is saying that the faster you straighten out, the more you should sideslip (it's the inverse of the second one). Again, vertical stabiliser (AKA a big tailfin, as far back as possible).

http://i1378.photobucket.com/albums/ah120/craigmotbey/Kerbal/Tuneups/screenshot117_zps880106ab.png

This is saying that there's a tendency to pitch-up as Z-direction (down) velocity increases. Keep in mind that a plane doesn't have to diving in order to have a high Z-velocity (actually, diving reduces your Z-velocity because of the reduced AoA). Because of the AoA required for lift, planes are always flying slightly down relative to the axis of the plane. Either add some more tailplane (horizontal stabilisers, again as far back as possible) or remove some lift from the front of the craft.

http://i1378.photobucket.com/albums/ah120/craigmotbey/Kerbal/Tuneups/screenshot118_zps5b9b3f02.png

[1] Sontaaw, I hope you don't take this the wrong way. It's not intended as a criticism of the original design; I just thought that it made a useful teaching moment. The original was much closer to flyable than most stock-aero based designs are.

The post was well done, and I'm man enough to take constructive criticism about things, which is why I came here to the forum SSTO guru. I'd actually set aside the OOPS shuttle plane aside while I got my station refit completed but I had already gone and added a vertical stabilizer. I'll have to poke through the rest of your advice and see where some of the already updated model might need to be tweaked (pretty sure I'd moved the lateral engines and wings forward a fair bit.

Edit: And looked back a page to see the OOPS Mk 3 kerbotuned, and love it. My FAR doesn't seem to be pulling up the same analyses as yours, so I'll have to fiddle so I can start looking at that important information. And save the jpegs or hell that entire post for reference material.

Edited November 28, 2014 by Somtaaw

[Wanderfound]

The post was well done, and I'm man enough to take constructive criticism about things, which is why I came here to the forum SSTO guru. I'd actually set aside the OOPS shuttle plane aside while I got my station refit completed but I had already gone and added a vertical stabilizer. I'll have to poke through the rest of your advice and see where some of the already updated model might need to be tweaked (pretty sure I'd moved the lateral engines and wings forward a fair bit.

It actually turned out really well after a fairly mild polish. Shifting the fuel weight forward, beefing up the tail (both vertical and horizontal; the original tailplane wasn't quite up to the job) and repositioning the wings to compensate was pretty much all it needed. There's a really nice plane in there; very fast, handles beautifully and plenty of practical usability.

With all of the aero surfaces, position relative to CoM is just as important as the structure of the surface itself. The further that the stabiliser or control surface is from CoM (in the relevant axis, so lateral for roll, longitudinal for pitch and yaw), the more effective it will be. This gives you three options when you need to increase stability: change the surface, move the surface, or move the CoM.

As well as reducing the CoM/dCoM offset, shifting the lateral tanks forwards increased the distance between the tailplane and CoM, thereby increasing the tail's leverage. This is also why I used the swept instead of straight tailplane on my version; the sweep increases the distance to CoM.

Pulling the lateral engines forwards required shifting the tailplane up out of the line of rocket thrust, in order to avoid the possibility of it getting torched by your own exhaust. The relatively small wings left it wanting a bit of dihedral for roll stability anyway, and doing that on the tailplane further removes it from danger.

The added intakes and pure LF supply aren't absolutely necessary, but they do let you get to orbit with more ÃŽâ€V. The only reason for the canards was because you seemed to want fairly rear-set wings; if you pull the wings up close to the intakes, the canards can be dispensed with.

[Wanderfound]

For vids have your given open broadcaster a try? It is free and while designed for streaming it can also record.

I think I tried that before without luck, but I'll give it another poke.

[Somtaaw]

Now the only tricky part is figuring out how to adjust my recently completed space station mark 2, code named Hephaestus, to include a good docking section for this beauty of a plane.

Now would you say I over muscled the base design, by going with 4 RAPIERs for the core weight?

Also, would you say that swapping out 2 of the RAPIER engines for say LV-909's or another variant of interplanetary engine would be easily done? Or would that require considerably redesign?

I'm a big fan of modular, so perhaps slide 2 docking clamps in likely onto the wings, for interchangable engine assemblies? I think in the end it might be very similar to your latest Impatienze design, just with twin expanded engines over the one.

[Hodo]

Now the only tricky part is figuring out how to adjust my recently completed space station mark 2, code named Hephaestus, to include a good docking section for this beauty of a plane.

Now would you say I over muscled the base design, by going with 4 RAPIERs for the core weight?

Also, would you say that swapping out 2 of the RAPIER engines for say LV-909's or another variant of interplanetary engine would be easily done? Or would that require considerably redesign?

I'm a big fan of modular, so perhaps slide 2 docking clamps in likely onto the wings, for interchangable engine assemblies? I think in the end it might be very similar to your latest Impatienze design, just with twin expanded engines over the one.

The biggest problem with that idea is the difference in mass of the two engines. The RAPIER is a pretty heavy engine for what you get, the LV909 is not. This would shift your CoM towards the nose of the craft throwing off the handling and balance of the aircraft. On the otherhand you have Aerospikes available which are almost the same weight and wont change the CoM that much if at all and have better ISP then the 909s or the RAPIERs. The other downside is the loss of half of your air breathing engine power will have an affect of increased climbing time, longer runways for take off and a much slower acceleration to speed for orbital burn.

Most cases I usually design a new craft for exclusively that reason just to take stuff to and from my space station at 100km orbit. The same SSTO also takes satelites and places them in Geosync orbit, pretty sad that it flew all the way to 2868.5km x 2868.5km orbit and managed to launch 2 satelites then return back to the KSC over shoot by 400km turn around then fly back to land.